The invention relates to packet-switched transmission of calls in a situation where the packets have to queue for forwarding, and particularly to packet-switched transmission of calls toward base stations on circuits between a telecommunication network and base stations in a mobile telephone system where a call is forwarded as data packets at even intervals.
In most digital mobile telephone systems the carrier wave of the radio path is divided between several users by using, for example, a TDMA (Time Division Multiple Access) or CDMA (Code Division Multiple Access) method. The common feature of the methods is that a different channel is allocated for each call, and that the call is divided into call packets of a certain length and the packets are transferred on different channels in successive frames transmitted on the carrier wave of the radio path. A frame comprises several channels. In order for a packet to arrive in time at the appropriate frame, the packet has to arrive in time at the base station. In the TDMA method, where a frame consists of time channels that are successive with respect to time, a call packet has to arrive at the base station in time so that it can be transmitted on the correct time channel.
In a packet-switched network the arrival rate and the order of arrival of the packets vary in the network node, for example, with the load of the network nodes encountered on the way, whereby packets arrive in random order, for example, at the network node located before the base station transmitting onto the radio path. The previously known network nodes transmit packets to the base station in the same order in which they arrive at the network node. The order of arrival may not be the appropriate order of transmission in view of the forwarding of the packets from the base station: the call packets must be transmitted onto the radio path at an even rate in the order of channels in accordance with the radio path protocol. The same disadvantageous situation may also arise when the network node transmitting the packets is connected to the rest of the network at a higher rate than the network node receiving the packets is connected to the transmitting network node. The packets will then have to queue at the network node for access to a lower-rate connection, and they are forwarded to the receiving network node in the random order of arrival.